Although lithium is widely distributed on Earth, there are very few commercial sources where lithium is found in concentrated values suitable for producing lithium compounds. One main source is the mineral spodumene, which is a double lithium-aluminum silicate, LiAl(SiO3), which has a theoretical lithium content of 8.03%. Other minerals which are exploited are petalite, LiAl(Si4O10), which has a theoretical content of Li2O of 4.88%, and lepidolite, which has a variable composition represented by the formula K2(LiAl)3(SiAl)4O10(OH,F)2. Another mineral of high potential value for obtaining lithium is hectorite, which is a low grade mineral with the composition Na0.33(Mg,Li)3Si4O10(OH,F)2 and which is not yet exploited for economic reasons.
Other sources for obtaining lithium, which have grown in importance in the last two decades, are the brines from salars, salt lakes, salt mines and geothermal resources. The highest lithium concentrations are found in salar deposits, which actually supply the majority of lithium to industry. The elements of major economic interest in salar type salt systems are lithium, potassium and boron. Important parameters for process development are:                a) the initial lithium concentration;        b) the Mg/Li ratio which reflects how much magnesium has to be removed by fractional crystallization or selective precipitation;        c) the Ca/Li ratio which indicates how much calcium has to be removed by fractional crystallization or selective precipitation;        d) the SO4/Li ratio which indicates whether the deposit is a lithium sulfate deposit (high SO4/Li) or a lithium chloride deposit (low SO4/Li). With a high ratio, lithium sulfate salts start to precipitate at lithium concentrations typically between 0.5% to 1%.        
In an evaporation process, calcium will concentrate together with magnesium and both have to be removed before lithium carbonate can be precipitated. When calcium concentrations are high, sulfate concentrations are low. Some typical components of naturally occurring brines are identified in the Table I below.
TABLE ISaline Brine Analysis (mg/l)ResourceLiKMgCaSO4BMg/LiSO4/LiCa/LiSalar de Atacama Average1,83522,62611,74137920,1807836.411.00.2Salar Salinas Grandes7759,2892,1171,4501,0362322.71.31.9Salar de Hombre Muerto7447,4041,02063610,2364201.413.80.9Salar de Hombre Muerto East Side7458,3181,7818,6422.411.6—Silver Peak2455,6553522137,576851.430.90.9Salar de Olaroz (production wells)7746,2272,00541618,6301,1362.624.10.5Salar de Cauchari6185,1271,77047619,1101,3602.930.90.8Salar de Uyuni Average4248,7197,87255710,34224218.624.41.3Salar de Rincón3977,5133,41949412,2093318.630.71.2Salar de Maricunga1,0368,8698,24711,9191,0956348.01.111.5
In the case of low Mg/Li brines, such as at Silver Peak, a process can be used in which magnesium is removed with slaked lime from fresh well brine or brine which is primarily concentrated in solar evaporation ponds. After the magnesium removal, the brine is concentrated to approximately 0.6% to 0.7% wt Li/wt brine, because at a higher concentration lithium sulfate salts will begin to precipitate. During the solar evaporation process principally sodium chloride together with some glaubersalt (Na2SO4.10H2O), glaserite (Na2SO4.3K2SO4) and gypsum (CaSO4.2H2O) are crystallized. In the more concentrated ponds, silvinite (KCl.NaCl) salts crystallize, which is feedstock for the production of potassium chloride (KCl). The concentrated lithium brine is purified by a boron removal step. The removal step can be completed by the addition of a mixture of calcium chloride and slaked lime, by solvent extraction of boric acid or by a brine polishing stage. The remaining magnesium and calcium are removed from the brine by adding (i) caustic soda or sodium ash, (ii) a recycled lithium carbonate end liquor, (iii) a mixture of slaked lime and soda ash or (iv) a combination of any two or more of (i)-(iii). Subsequently, the brine is filtered and heated between 60 to 90° C. and a soda ash solution is added to precipitate lithium carbonate. The precipitated lithium carbonate is filtered washed and dried. This process produces a technical grade lithium carbonate (typically 99.0% wt Li2CO3) which is not suitable for the more exigent lithium battery industry or for pharmaceutical compositions that contain lithium carbonate.
At the Salar de Hombre Muerto, which is also a low Mg/Li brine, the brine is conditioned to the appropriate pH and temperature before it enters a sorption-desorption process where lithium, as lithium chloride (LiCl), is selectively recovered. The pregnant liquor contains approximately 0.16% wt Li/wt brine. Afterwards, the effluent is sent to the solar evaporation ponds where it is purified, concentrated and pH adjusted. If the brine reaches a concentration of 3% % wt Li/wt brine, it is sent to the lithium carbonate process, where boron is removed with selective ion exchange. The remaining magnesium and calcium are removed from the brine with the addition of (i) sodium hydroxide and sodium carbonate, (ii) recycled lithium carbonate end liquor, (iii) a mixture of slaked lime and soda ash or (iv) a combination of any two or more of (i)-(iii). The brine is filtered and heated between 60 to 90° C. and a soda ash solution is added to precipitate lithium carbonate. The precipitated lithium carbonate (typically 99.5% wt Li2CO3), is filtered washed and dried. This process produces a battery grade of midrange purity.
The brines with higher Mg/Li ratios, such as at the Salar de Atacama, are pumped from the salar reservoir and are evaporated in large shallow pools, where a sequential crystallization of the salts commences. As the chloride brines are generally saturated with sodium chloride, the first salt to be precipitated is halite with some gypsum, followed by silvinite crystallization. Further evaporation leads to carnalite (KCl.MgCl2.6H2O) and then bischofite (MgCl2.6H2O) crystallization. At this stage, the lithium is increased to about 4.5% wt Li/wt brine with a magnesium content of about 4%. The evaporation of the brine continues forcing the crystallization of lithium carnalite (LiCl.MgCl2.6H2O), which is leached with less concentrated brine to recover part of the lithium content. The final lithium chloride brine contains about 6% wt Li/wt brine, 1.8% wt magnesium/wt brine, 0.8% wt boron/wt brine and some minor amounts of potassium, sodium and sulfate. Subsequently, the boron is removed as boric acid by solvent extraction. After the boron extraction, the brine is mixed with the lithium carbonate plant end liquor in order to precipitate most of the magnesium as magnesium carbonate. The brine is filtered and sent to the second stage of magnesium and calcium removal. In this second stage, a mixture of slaked lime and soda ash is added to the filtered brine in order to precipitate the magnesium as magnesium hydroxide and the calcium precipitates as calcium carbonate. The resulting sludge is filtered and the purified brine is sent to the lithium carbonate stage. Soda ash solution is added to the purified lithium brine in order to precipitate lithium carbonate. This reaction takes place at elevated temperatures normally between 60 to 90° C. Subsequently, the lithium carbonate (typically 99.4% wt Li2CO3), is filtered and the cake washed on a belt filter and finally dried. This process produces a battery grade of low range purity.
New brine deposits, with low Mg/Li and low sulfate brines (low. SO4/Li), such as Salinas Grandes or relatively low sulfate brines as at the East Side Salar de Hombre Muerto, the lithium can be concentrated up to 6% wt Li/wt brine. Other brine deposits with high Mg/Li and high SO4/Li, such as the sulfate brine in the Salar de Atacama, Salar de Uyuni, East and West Tanjinair Salt Lakes in China, and those with high Ca/Li brines such as Salar de Maricunga in Chile currently lack industrial application. Nevertheless, various process developments attempt to generate concentrated lithium brines suitable to precipitate lithium carbonate.
One process development involves using an additional process in which an impure feed of lithium carbonate (Li2CO3) is mixed with an aqueous solution and reacted with CO2, preferably under pressure, to produce dissolved aqueous lithium bicarbonate (LiHCO3). Insoluble impurities such as iron, magnesium and calcium are removed by physical means such as filtration or centrifugation. Soluble divalent or trivalent ions such as magnesium, calcium and iron are adsorbed by selective ion exchange or other similar methods. Carbon dioxide is then completely or partially removed by raising the solution temperature and/or releasing the pressure to enable pure Li2CO3 to precipitate. A part of the solution is returned to the bicarbonation reaction zone to enhance the economics of the process. Undesirable impurities remain in solution.
Although there are many ways to produce lithium carbonate from lithium containing brine, no simplified process exists to produce lithium carbonate of high purity, such as battery or pharmaceutical grade, from concentrated lithium brines that contain significant amounts of other values such as boron, magnesium, calcium, sodium, potassium, chloride and sulfate. Typical values for battery grade or pharmaceutical grade lithium carbonate are shown in Table II.
TABLE IITypical values for Battery Grade or Pharmaceutical Grade Lithium CarbonateBattery GradeBattery GradePharmaceutical(low)(high)GradeLi2CO399.2099.6099.70Na0.0600.0180.003Fe0.0020.00010.0003Ca0.0400.0050.01SO40.1000.0200.01K0.0100.00040.001Cl0.0100.0020.001H2O0.4000.1930.05Mg0.0100.0020.001Cr0.00100.0002NDNi0.00300.0002NDCu0.00100.0002NDPb0.0020.002NDHeavy Metal (Pb)0.00200.00200.0002AsNDND0.0002Al0.0050.0003NDZn0.0050.0007NDB0.0010.0002NDMn0.00050.0003NDSi0.0050.0010NDF0.0100.0050NDInsol. In HCl0.00300.00200.00LOI (550° C.)0.0100.0050NDND = No Data available